Welcome to the Wilmington North Carolina National Weather Service science and technology page.
On this page you will find comprehensive explanations of local and regional weather phenomena that
make forecasting for the area a challenge. Use the quick links below to navigate to the section
you are interested in.

The sea breeze can have a dramatic affect on temperatures along and sometimes well inland of
the immediate coast. The sea breeze can also act as a "trigger" for thunderstorms.

Forecasters at The National Weather Service in Wilmington have classified several types of
sea breezes through the years. Click on the images below to get a closer look at the
radar and satellite configurations. Many of the loops will be posted as they become available.

The classic sea breeze occurs when a weak pressure gradient is present. The shape
closely resembles the coast. There is always an inflection point with the
classic sea breeze. It is often accompanied by weak showers and thunderstorms.
The classic sea breeze can propogate well inland.

Classic Sea Breeze

The Southerly Resultant Sea Breeze with Inflection is one of the most common types of seabreezes in our area
due to the prevailing flow. This feature always has an inflection point which is displaced to the
north or northeast. It generally stays confined to the coastal counties and can be accompanied
by weak pulse storms.

Southerly Resultant Sea Breeze With Inflection

The Southerly Resultant Segmented Sea Breeze partially mimics the coastline but stays
close to the coast across the southern areas. There is no inflection point. This
sea breeze often has little in the way of convection because the atmosphere is capped.

Southerly Resultant Segmented Sea Breeze

The Southerly Resultant Hybrid Sea Breeze remains confined to the coast across the
northern and southern areas while moving inland toward the center.

Southerly Resultant Hybrid

The Northeast Resultant Sea Breeze with Inflection slightly mimics the coast.
This feature always has an inflection point that is displaced to the west. The
inland propogation is limited...and with the prevailing Northeasterly flow...convection is usually
limited because of poor instability.

Northeast Resultant Sea Breeze with Inflection

The Northeast Resultant Segmented Seabreeze partially mimics the coast and remains confined
near the coast along the northernmost segments. This feature does not have an inflection point
and usually inland propagation is limited.

First, CAPE is observed over the entire sounding. Zero CAPE gives no correlation...up to 1200 J/kg
gives moderate correlation and 1200 to 2000 J/kg gives a high correlation.

The distribution of instability is alos important. Waterspouts require instability to be
present in the lower portion of the troposphere where convective vertical velocities
can be wrapped into a waterspout's circulation. This is measured by comparing
temperatures at 900mb and 600mb. The greater the difference in these temperatures
indicates a larger amount of potential instability. This is true to a point...however very
steep lapse rates nearing dry adibatic in the layer can occur when the
atmosphere is too dry to support convection or waterspouts.

Analyze the winds in the sounding at the 1000...2000...4000 and 6000 foot agl layers. Shear
between each of these layers can be computed by doing a vector subtraction of the u and v
components of the winds at each level. The resultant wind...i.e. the shear...is an indication of
how much distortion a rising updraft and growing cumulus cloud will enconter as it
ascends. Waterspouts...despite their occasionally extreme wind speeds...are very fragile
and can be disrupted by high values of wind shear.

The following chart shows values of
wind shear which have been found to be correlated with waterspout formation:

Since waterspouts are tied so closely to moist convective processes, the presence of
sufficient moisture in the atmosphere is obviously important. Analysis of three years of
soundings on days with waterspouts has revealed moist air(characterized by dewpoint
depressions < 5 C) should extend from the surface up to 600mb.

A daily rip current outlook is included in the Surf Zone Forecast which
is issued by many National Weather Service offices...including Wilmington North Carolina. A three-tiered structure
of low, moderate, high is used to describe the rip current risk. This outlook is communicated to
lifeguards, emergency management, media and the general public.

Just like the Waterspout Risk Calculator...there are several components that go into
derivine the rip current risk. Click on the links to the right of the image below
to read a description of the component.

Wind direction is very important to the development of strong rip currents. With the orientation of our
beaches in this area...a subtle change in wind direction can have significant impacts
on the development and intensity of rip currents.

For the Southeast facing beaches...like the ones in
Pender...Horry...and Georgetown counties...a wind direction of 80 degrees to 170 degrees is important. For the
beaches in New Hanover county...which face more to the east...a wind direction of 70 degrees to
160 degrees is important. For the Brunswick County beaches...which face more to the south...a wind
direction of 150 degrees to 220 degrees is important.

The swell component in the wave spectrum also plays a significant role in the development and intensity
of rip currents. The longer period swells...like the ones that eminate from a distant hurricane...
provide more power by pushing more water on to the beach. In general...the swell direction must
be normal or perpindicular to the beach or county in question.

Tides play a significant role in the development of rip currents. The gravitational
influence of the earth's moon is stronger three days either side of a full or new moon. More
weighing is given to the scheme during these periods.
Also studies have shown the frequency of rip currents is higher during low tide so
more weighing is alos given if an extremely low tide occurs durng typical beach
going hours.

The synoptic flow plays a role in the development of significant rip currents. Once again the direction
is important for the respective beaches...with the southeast facing beaches favoring the
direction range of 80 and 170 degrees. The south facing beaches favoring the direction range of
150 amd 220 degrees with the east facing beaches favoring a direction range of 70 and 160 degrees.
The longevity of the flow is important as well. Not only does the direction have to be in the
favored range...the flow must have occurred 80% of the time during the previous 48 hours.

African dust is generally thought to be an inhibiting factor for the development of tropcial systems in the Atlantic.
It is associated with very dry air at the mid levels of the atmosphere. Tropical systems develop and intensify in
areas where the moisture content is high. The Saharan Air can be analyzed at
The Cooperative Institute for Meteorological Satellite Studies...CIMMS.

Sea Surface Temperatures/Tropical Cyclone Heat Potantial

Warm water is a necessary component to tropical storm development. Water temperatures of 80 degrees
Fahrenheit or about 26.5 degrees Celsius are the thresholds. A good site to view water temperatures
can be found at the Tropical Cyclone Heat Potential Page.
The depth of the warm water...referred to as the heat potential...is also critical and can be examined at the same page.

Basically...wind shear refers to any change in wind speed or direction along a straight line.
In the case of hurricanes, wind shear is important primarily in the vertical direction--from
the surface to the top of the troposphere. Wind shear of less than 10 knots is very condusive to
tropical cyclone formation. Increasing values of wind shear are less condusive for rapid development and values
exceeding 20 knots are usually destructive. Once again...there are a couple of excellent sites to view current
and forecast wind shear. The first is
CIMMS. The Penn State site is anothe good reference. You need to
select the 850-200MB parameter on the GFS model and step through the six hour panels.
Jeff Masters of The Weather Underground has an excellent
wind shear tutorial.

SLP/1000-500mb ThicknessThis parameter is based on the difference in heights between 2 pressure levels.
h=z1-z0
From the hypsometric equation one can theoretically derive the thickness base on a mean temperature for a layer.
h=(R/g)*Tmean*ln(P0/P1)
Where R = gas constant
g = acceleration of gravity
Tmean = mean temperature for the p0 to p1 layer.
Since R and g are constants, then for any given layer the thickness is proportional to
the mean temperature of the layer. Thus the cooler the layer the less thick the layer is.
This parameter is used for determining the thermal structure of a front, delineation of rain,
snow or freezing precipitation, and also the movement of an MCS.

925mb Moisture TransportMoisture Transport is a derived parameter of the model wind field and
the moisture field. The values displayed multiple the two fields. This field shows where moisture
is advecting. These fields are best from 700 mb and lower.

850mb Height/RH/OmegaUpper vertical motion is closely
connected to the advection of vorticity in the upper troposphere and warm air advection in the lower
troposphere. The Omega equation based on these advections
can approximate the vertical motion in the atmosphere. The depiction of a negative omega
indicates areas of upper vertical motion and positive omega represents downward vertical motion.
Typically users have used the 700 mb level to diagnosis vertical motion on the synoptic scale
but other levels are just as valuable such as 850 or 500 mb based on your elevation.

850mb Moisture TransportMoisture Transport is a derived parameter of the model
wind field and the moisture field. The values displayed multiple the two fields.
This field shows where moisture is advecting. These fields are best from 700 mb and lower.

700mb Height/Temp/WindThe 700mb level is a standard level
in the atmosphere...approximately 9k feet above the surface. This level is used to analyze
shortwave troughs and ridges.

700mb Height/RH/OmegaUpper vertical motion is closely
connected to the advection of vorticity in the upper troposphere and warm air advection in the lower
troposphere. The Omega equation based on these advections
can approximate the vertical motion in the atmosphere. The depiction of a negative omega
indicates areas of upper vertical motion and positive omega represents downward vertical motion.
Typically users have used the 700 mb level to diagnosis vertical motion on the synoptic scale
but other levels are just as valuable such as 850 or 500 mb based on your elevation.

700mb Moisture TransportMoisture Transport is a
derived parameter of the model wind field and the moisture field. The values displayed
multiple the two fields. This field shows where moisture is advecting.
These fields are best from 700 mb and lower.

500mb Height/Temp/Wind
Probably the most common used mid level standard level in the atmosphere...
approximately 18k feet above the surface. This layer is used to analyze short and long wave
troughs and ridges...jet streams.

500mb Height/RH/OmegaUpper vertical motion is closely
connected to the advection of vorticity in the upper troposphere and warm air advection in the lower
troposphere. The Omega equation based on these advections
can approximate the vertical motion in the atmosphere. The depiction of a negative omega
indicates areas of upper vertical motion and positive omega represents downward vertical motion.
Typically users have used the 700 mb level to diagnosis vertical motion on the synoptic scale
but other levels are just as valuable such as 850 or 500 mb based on your elevation.

500mb Height/VorticityIn meteorology vorticity
refers to the measure of rotation or spin of the atmosphere. This field when viewed on a constant
pressure surface when assuming adiabatic flow can help determine upward and downward vertical motion on a synoptic scale. In an Eulerian framework, one where we can watch the change of vorticity at a fixed point one can sense whether the value of the vorticity is increasing or decreasing with time. If it is increasing then this is positive vorticity advection and one can infer upward vertical motion at this point. The opposite is true for negative vorticity advection.
500 mb level is used for this assumption as it is near the level of non divergence.

300mb Wind SpeedAn important standard level in the atmosphere
ranging from 30 to 35k feet...depending on the season. This field is used to analyze thermally direct and
indirect circulations of ageostrophic circulaitons of jet streaks as well as long wave troughs and
ridges.

250mb Wind SpeedAn important standard level in the atmosphere
ranging from 35 to 40k feet...depending on the season. This field is used to analyze thermally direct and
indirect circulations of ageostrophic circulaitons of jet streaks as well as long wave troughs and
ridges.

Precipitable Water/SLPPrecipitable Water, PW, is the
total atmospheric water vapor contained in a vertical column extending between any two specified levels, generally
from the ground to the top of the atmosphere or the top of an atmospheric model. PW is used to determine the abundance or lack of moisture. There are 3 factors which are needed for convection to form; these three factors are the availability of instability, upward vertical motion, and available moisture.
PW is one way a meteorologist monitors the moisture in the atmosphere.

CAPE/SLP/10m Windis the amount of energy
a parcel of air would have if lifted a certain distance vertically
through the atmosphere. CAPE is effectively the positive buoyancy of an air parcel and is an
indicator of atmospheric instability, which makes it valuable in predicting severe weather.

Best CAPE/CINIs equivalent to the Most Unstable CAPE which is a measure on
instability in the tropopause. The Best Cape is calculated by averaging the potential temperature
and water vapor mixing ratio in the lowest levels of the model. The most unstable layer
is chosen and this parameter should be used to help diagnose where the most unstable air is.

Best LIThe Best Lifted Index is derived from
calculating the Lifted Index from each of the "boundary" layers in the model and taking the
most unstable or "Best" Lifted index. Note: Best LI does not consider such things as the
capping inversion which could limit or prevent the convection from developing.
This parameter should be used to help diagnose where the most unstable air is.

1 Hour Precipitation
The one hour precipitation accumulation developed from the forecast model.

6 Hour Precipitation
The six hour precipitation accumulation developed from the forecast model.

12 Hour Precipitation
The twelve hour precipitation accumulation developed from the forecast model.

Composite ReflectivitySimulated Composite Reflectivity has
a great advantage in that it displays detailed mesoscale and near-stormscale structures
capable of being forecast by finer resolution models, such as lake-effect snowbands, the structure
of deep convection, and frontal precipitation bands. (Koch, 2005)

Freezing Level

CloudsThe effects of the clouds are
entirely prognosed from predicted fields: water vapor, cloud water and ice, and frozen
and liquid precipitation. Values are given in percent where 0% is totally clear skies
and 100% is totally overcast.